Implement style computing

[TrySound]

Ref #777

Currently a lot of optimisations are attributes specific and may be
broken because of inline or shared styles.

In this diff I'm trying to solve the problem with getComputedStyle
analog.

computeStyle collects attributes, shared css rules, inline styles
and inherited styles and checks whether they can be statically optimised
or left as deoptimisation.

As an example I integrated computeStyle into removeHiddenElems plugin
which is fixed bugs with styles in style rules and inline styles.
Note: review with hidden whitespaces

[TrySound]

cc @strarsis @XhmikosR @caub @sk-

[strarsis]

The inlineStyles plugin already does this, it collects the styles and then stably sorts them which surprisingly is sufficient. Just selector-based filtering would be needed to compute the styles of a particular element, which is also easily possible now. It should be possible to extract this part into a dedicated function (css tools module). As a performance optimization it would be also possible to provide a prepared presorted structure as the plugin does so this doesn't has to be performed each time. Alternatively transparent caching could be used (as it was added for element styles support).

[TrySound]

Proper caching will require some architecture changes. First we should fix bugs.

And yeah, I saw inlineStyles though the point here is resolving style from all sources including inheritance and presentation attributes.

All plugins should work correctly in isolation. So caching will be implemented on core level not in plugin.

[caub]

looks really good 👍🏽 from the tests, never thought getComputedStyle could be done in this quite short way

[TrySound]

Me neither 😄